1. Field of the Invention
The invention relates to a hydraulic control system that is applied to a belt-type continuously variable transmission that changes the belt turning radii of a drive pulley and a driven pulley, around which a belt is wound, on the basis of sheave pressures applied respectively to the pulleys to shift gears, wherein the hydraulic control system performs hydraulic control to regulate the sheave pressures.
2. Description of the Related Art
A belt-type continuously variable transmission is in practical use as a transmission for a vehicle, or the like. For example, as shown in FIG. 4, the belt-type continuously variable transmission includes a drive pulley 100 and a driven pulley 101, of which the respective groove widths are variable, and a metal belt 102 wound around these pulleys. Then, the groove width of each of the drive pulley 100 and the driven pulley 101 is varied to change the turning radius of the belt 102 on those pulleys, thus steplessly shifting gears.
The groove widths of the drive pulley 100 and driven pulley 101 of the belt-type continuously variable transmission are varied by regulating hydraulic pressures (a first sheave pressure Pin and a second sheave pressure Pout) applied respectively to the pulleys. Therefore, the belt-type continuously variable transmission is equipped with a hydraulic control system that performs hydraulic control to regulate the sheave pressures.
As shown in FIG. 4, in the hydraulic control system of the belt-type continuously variable transmission, hydraulic oil supplied from a pump is regulated in pressure at a first regulator valve 111 and a second regulator valve 112 to thereby obtain a line pressure Pl, which is used for hydraulic control as a source pressure. In addition, the line pressure Pl is reduced in pressure at a modulator valve 113 to obtain a constant modulator pressure Pm.
The modulator pressure Pm output from the modulator valve 113 is supplied to a first solenoid valve 114 and a second solenoid valve 115. The first solenoid valve 114 and the second solenoid valve 115 respectively regulate the modulator pressures Pm by controlling an electric current supplied to respective internal linear solenoids to thereby obtain and output a desired first solenoid pressure Pslp and a desired second solenoid pressure Psls.
The first solenoid pressure Pslp output from the first solenoid valve 114 is transmitted to a first sheave pressure regulating valve 116. Then, the first sheave pressure regulating valve 116 regulates the line pressure Pl on the basis of the first solenoid pressure Pslp to obtain a first sheave pressure Pin applied to the drive pulley 100. On the other hand, the second solenoid pressure Psls output from the second solenoid valve 115 is transmitted to a second sheave pressure regulating valve 117. Then, the second sheave pressure regulating valve 117 regulates the line pressure Pl on the basis of the second solenoid pressure Psls to obtain a second sheave pressure Pout applied to the driven pulley 101. Thus, in the hydraulic control system, the first sheave pressure Pin and the second sheave pressure Pout, which are applied respectively to the drive pulley 100 and the driven pulley 101, are regulated by controlling electric currents supplied respectively to the linear solenoids of the first solenoid valve 114 and second solenoid valve 115 to thereby variably set the groove widths of the drive pulley 100 and driven pulley 101. By so doing, the gear shift control of the transmission is performed.
In the hydraulic control system of a continuously variable transmission, in order to improve the control accuracy of the first sheave pressure Pin and the second sheave pressure Pout, an output pressure may possibly be regulated at the first sheave pressure regulating valve 116 or at the second sheave pressure regulating valve 117 in a feedback manner.
The output pressure from the first sheave pressure regulating valve 116 is regulated in a feedback manner as follows. Here, it is assumed that a gain of the output pressure (first sheave pressure Pin) against the command pressure (first solenoid pressure Pslp) at the first sheave pressure regulating valve 116 is “α1”. In the first sheave pressure regulating valve 116 at this time, a force corresponding to a differential pressure (Pin−α1×Pslp) between the first sheave pressure Pin (output pressure) and the product (α1×Pslp) of the first solenoid pressure Pslp (command pressure) and the gain al acts on the valve element. Then, the first sheave pressure Pin is regulated in a feedback manner so that the valve element moves to reduce the differential pressure and thereby the first sheave pressure Pin (output pressure) becomes a value corresponding to the first solenoid pressure Pslp (command pressure). Thus, it is possible for the output pressure to quickly follow the command pressure in the first sheave pressure regulating valve 116 and, as a result, it is possible to improve the control accuracy of the first sheave pressure Pin, that is, it is possible for an actual gear ratio to quickly follow a target gear ratio of a continuously variable transmission.
Incidentally, the first sheave pressure regulating valve 116 and/or the second sheave pressure regulating valve 117 may possibly produce fail so that the output sheave pressure becomes relatively high, for example, due to entrapment of foreign matter, or the like. When the above failure that the sheave pressure becomes excessive, that is, a failure due to an excessive sheave pressure, occurs, the line pressure Pl that is regulated by that sheave pressure also increases with the increase in sheave pressure. Then, the sheave pressure further increases with the increase in line pressure, and then the line pressure further increases with the increase in the sheave pressure. In this way, a vicious cycle occurs, and the sheave pressure continuously increases. This may deteriorate the durability of the belt 102. In addition, the excessive increase in line pressure Pl deteriorates the durability of the oil pump.
As described, for example, in Japanese Patent Application Publication No. 2005-163869 (JP-A-2005-163869), a hydraulic control system of a continuously variable transmission is described, which is equipped with a fail-safe valve to prevent the above described excessive increase in hydraulic pressures applied respectively to the pulleys at the time of a failure due to an excessive sheave pressure. As shown in FIG. 5, in this hydraulic control system, a fail-safe valve 120 is provided between the first sheave pressure regulating valve 116 and the drive pulley 100. The fail-safe valve 120 normally outputs the first sheave pressure Pin output from the first sheave pressure regulating valve 116 to the drive pulley 100, while the fail-safe valve 120 outputs the second sheave pressure Pout output from the second sheave pressure regulating valve 117 to the drive pulley 100 at the time of a failure due to an excessive first sheave pressure Pin.
Incidentally, to increase the gear shift speed of the continuously variable transmission, it is necessary to improve the response of the first sheave pressure regulating valve 116 and the response of the second sheave pressure regulating valve 117. When the output pressures (the first sheave pressure Pin and the second sheave pressure Pout) are regulated in a feedback mariner, the control accuracy of each output pressure is improved; however, the speed of response of each output pressure to the command pressure (the first solenoid pressure Pslp and the second solenoid pressure Psls) may slightly decrease. For this reason, there has been a limit to improve the speed of response while ensuring the control accuracy of each sheave pressure.